LAMBERT: CONSTITUTION OF THE EARTH 13I 



work that the precession of a soHd earth would be practically 

 indistinguishable from that of a fluid one. 



The first real evidence that the earth acts like an elastic solid 

 rather than like a mass of fluid — at least in respect to forces acting- 

 over a short time only — came from the tides. The tides raised 

 by the sun and the moon can be decomposed into partial tides 

 falling into three classes: (i) Partial tides whose period is 

 nearly a day; (2) partial tides whose period is nearly half a day, 

 and (3) the so-called long-period tides whose period is nearly a 

 fortnight or a month for the moon and six months or a year for 

 the sun. With our present mathematical knowledge, we are 

 utterly unable to predict, without recourse to observation, the 

 tides of the first and second classes, the so-called diurnal and semi- 

 diurnal tides, which are conveniently lumped together under one 

 heading as the short-period tides. The diagrams in most text- 

 books on astronomy intended to explain the origin of the tide 

 generating forces almost inevitably suggest the idea that the tidal 

 swelling travels round the earth keeping pace with the m.oon 

 and remaining directly under it. Sometimes this statement is 

 made in so many words, with qualifications for the effect of land 

 barriers. This gives a very incorrect idea of the mechanics of 

 the problem as far as it concerns the diurnal and semi-diurnal 

 tides. Expressed in more mathematical form, the idea is that 

 the ocean adjusts itself to the forces so that its surface is always 

 an equipotential surface for the instantaneous field of force. 

 The ability of the ocean to adjust itself thus to the forces de- 

 pends on the depth and the period of the forces, and our actual 

 ocean is far too shallow to adjust itself even approximately to the 

 tidal forces with periods of half a day or a day. It appears to 

 be otherwise with the long-period tides. 



Lord Kelvin assumed that we can predict the amount of their 

 rise and fall for the case of a rigid earth from the known masses 

 and distances and positions of the sun and moon, because as far 

 as these tides are concerned, the time is sufficient to allow the 

 ocean surface to become an equipotential surface for the tide- 



